This invention relates generally to apparatus for sensing linear acceleration. More particularly, this invention pertains to a seismic sensor that includes a fiber optic interferometer arranged to provide an optical signal in response to seismic vibrations.
The measurement of flexure or deformation of an elastic disk in response to acceleration or pressure comprises the operating principle of numerous acceleration and pressure sensors. The amount of deformation or displacement can be determined interferometrically, mechanically, piezoelectrically or by changes in the capacitance or inductance between elements. However, all such systems have deficiencies such as limited sensitivity, high cost, limited maximum deflection and operating environment sensitivities. Some errors due to operating conditions are fundamental, such as limited physical flexure capacity in response to acceleration that renders the desired output indistinguishable from signal components associated with noise sources. Other operating condition errors can result from changes in physical dimension, modulus of elasticity, index of refraction, etc., occasioned by temperature and pressure changes.
Interferometric strain measurements exhibit superior accuracy and resolution. When carried out by means of an optical fiber, interferometric systems include simple and rugged sensor devices with low power requirements, immunity to electromagnetic interference and ready adaptability to remote sensing and high data rates. Interferometric measurements of acceleration and pressure employing an optical fiber medium can be accomplished through telemetric signal transmission of a multitude of sensors in a single fiber using time division multiplexing. The fibers are themselves relatively insensitive per unit length and not subject to errors due to ambient pressure, tension from acceleration, etc.
A number of acceleration measurement devices have been developed that utilize disk-mounted spiral coils of optical fiber to produce a desirable push-pull effect. U.S. Pat. No. 4,959,539, issued Sep. 25, 1990 to Hofler et al., discloses a hydrophone having a disk circumferentially supported for flexure induced by acoustic vibrations. A flat spiral of optical fiber is fixed to each side of the disk and arranged as the two legs of an interferometer. Flexure of the disk shortens the optical path length of the spiral on one surface while lengthening it on the oppositely-facing surface. A pair of the disks may be mounted on a body so that an acoustic pressure differential to be measured exists across the disks with the spirals being connected for push-pull operation as two legs of a fiber optic interferometer.
U.S. Pat. No. 5,317,929, issued Jun. 7, 1994 to Brown et al., discloses a fiber optic accelerometer based upon the double disk structure described above. A centrally-located mass is clamped between the opposed flexible disks. Flat spiral coils of optical fiber are fixed to the surfaces of flexible disks and are arranged to be included in the legs of an interferometer.
U.S. Pat. No. 5,883,308, issued Mar. 16, 1999 to Fersht, one of the present inventors, discloses a fiber optic accelerometer that includes a ring that twists in response to an acceleration that is orthogonal to the plane of the ring. The ring is mounted by means of a peripheral flange to the walls of a casing. The casing has two concentric walls that divide the interior of the casing into a disk-like central section and an annular surrounding section. The ring is arranged to have a moment of inertia that causes the ring to twist in response to linear acceleration along the input axis. Flat spiral optical fiber coils are fixed to the top and bottom of the ring and are arranged to be included in the legs of an interferometer for generating an optical signal that may be processed to determine the acceleration. The central portion of the casing is hollow to make the device neutrally buoyant so that it is suitable for use in under-water acoustic sensing applications. The disclosure of U.S. Pat. No. 5,883,308 is hereby incorporated by reference into the present disclosure.
A fiber optic seismic sensor according to the present invention comprises a central support assembly that is preferably formed of a metal such as aluminum. A support plate has an inner portion retained within the central support assembly and an outer portion that extends beyond the central support assembly. A first substrate has an inner end mounted on a first side of the outer portion of the support plate. The first substrate is formed generally as a hollow cylinder with an inner radius and an outer radius and an inner wall and an outer wall with the inner radius being dimensioned to receive the central support assembly inside the first substrate such that the central support assembly and the inner wall of the first substrate are spaced apart. A second substrate has an inner end mounted on a second side of the outer portion of the support plate. The second substrate is formed generally as a hollow cylinder with an inner radius and an outer radius and an inner wall and an outer wall. The inner radius is dimensioned to receive the central support assembly inside the second substrate such that the central support assembly and the inner wall of the second substrate are spaced apart. A first spiral-wound optical fiber coil is formed on an outer end of the first substrate and a second spiral-wound optical fiber coil is formed on an outer end of the second substrate. The first and second optical fiber coils are arranged such that they are generally flat, concentric and spaced apart. An interferometer is formed to include the first spiral-wound optical fiber coil in a first leg and the second spiral-wound optical fiber coil in a second leg. The interferometer is arranged such that acceleration along an axis perpendicular to the planes of the optical fiber coils causes a deflection in the support plate, which causes an increase in the length of one of the optical fiber coils while causing a corresponding decrease in the pathlength of the other optical fiber coil.
The central support assembly preferably comprises a first central support member having a cavity in a first end thereof, and a second central support member having a projection extending therefrom and arranged to be received within the cavity in the first central support member. The support plate includes a passage arranged to receive the projection therethrough such that the inner portion of the support plate is retained between the first and second central support members.
An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawings.